1. Field of the Invention
The present invention relates to an optical fiber. More particularly, it relates to an optical fiber which has a protective coating with high strength and high thermal resistance.
2. Description of the Related Art
An optical fiber glass easily breaks because of flaw when it has no coating. Therefore, the glass is coated by a thermosetting, ultraviolet light curing or thermoplastic resin to make a protected optical fiber which is used as a light-guiding medium in an optical fiber cable.
By the way, the optical fiber is used in various fields. It is desired to use the optical fiber under specific circumstances other than usual circumstances. Particularly, it is required to develop the thermally resistant optical fiber which can be used under the circumstances subjected to high thermal energy or radiation energy, such as in an oil well mining apparatus, a power and light composite cable, a cable in artificial satellite. To this end, ladder type polyorganosiloxane is attractive as a thermally resistant coating resin material.
However, in addition to the requirement of thermal resistance in use, the protective coating must have suitable elongation in view of handling of the optical fiber. The coating of such ladder type polyorganosiloxane and other resin cannot satisfy the thermal resistance and the elongation simultaneously. If the thermal resistance is sufficient, the protective coating has low elongation and then the optical fiber cannot be practically used, and vice versa.